The invention starts out from an optical indication and operation monitoring unit according to the preamble of the main claim. In a known device of this type (German Disclosure Document No. 34 31 164), optical dust detecting means, namely a light transmitter in the form of a luminous diode (LED) and a light receiver, for example a phototransistor, are arranged in the suction channel of a vacuum cleaner, which is passed by the dusts, solid particles, or the like drawn in, in such a manner that optical indication lamps are actuated by the interruption, dispersion or reflection of the light beam and by amplification of the electric signal emitted by the photo transistor, the arrangement of the optical indication lamps being such that, for example, a green luminous diode lights up when practically no dust is detected while a red luminous diode lights up when dusts, solid particles, or the like, pass the suction channel. In this manner it is possible to monitor the operation of the vacuum cleaner in a particularly efficient manner because when assessing the degree of cleanliness already reached during the cleaning process the user of the vacuum cleaner is no longer bound to rely on his own personal judgement, but has available an objective indication whether or not a sufficiently high degree of cleanliness has been reached in the area just worked with the vacuum cleaner. The known device uses a delay element connected to the output end of amplifier means for the phototransistor, which delay element is triggered every time dust is detected and which may, for example, consist of a monoflop which remains in its unstable condition for a predetermined period of time, for example two or three seconds, during which time a, for example, red luminous diode lights up to indicate the presence of a corresponding amount of dust. When no further dusts or solid particles are detected by the optical sensors in the suction duct of the vacuum cleaner, then the active time of the monoflop runs out, the green luminous diode lights up, and the user of the vacuum cleaner can proceed to the next area, for example of a carpet. Although this arrangement permits to carry out the cleaning operation with a minimum of energy, cost and time, as it prevents the user from working the areas to be cleaned unnecessarily long, with the resulting excessive degree of wear, it is a problem in connection with the current supply of such an appliance, which in the case of a cylinder vacuum cleaner would have to be effected through the connection hose, that the optical means may get blocked, for example, by very fine dust particles in the suction duct and indicate a degree of cleanliness which actually has not been reached yet. For, when no dust is detected by the optical means, this may have two causes: either there may actually be no dust in the suction channel, or the active surfaces of the luminous diode and/or the phototransistor may have been blocked.
On principle, it has been known to evaluate the amount of dusts detected in the suction pipe of a vacuum cleaner by optical means, namely a photoelectric cell and a light source, for regulating the operation of a vacuum cleaner (German Disclosure Document No. 23 36 758). But except for the information that a light effect detected by a photoelectric cell through dispersion in the suction duct imparts a corresponding signal to suitable control means which then regulates the motor speed of the vacuum cleaner, no other indications can be derived from the cited publication, in particular as regards the practical operation and evaluation of the signals obtained.
Now, it is the object of the present invention to provide an optical indication and function monitoring unit for a vacuum cleaner which can be fitted, maybe even retrofitted, in or on any vacuum cleaner independently and separately and which, in addition to the two basic dust-indication functions, is capable of monitoring and indicating, with a minimum of input, quite a number of additional functions of the vacuum cleaner without the need to switch such an indication and function monitoring unit on manually.